IDEA does surveys on instructor salaries. Here is a link to an article with some info you might find interesting. Specialty instructors is a broad category, but gives you some idea.
You do not say where you are, but remember that this is going to vary widely geographically. You might start googling other Tai Chi teachers in your area and see what a range of published prices are.
Also remember that the rates will differ if you are teaching in a health club, a stand alone studio, in a corporate setting, or as your own business. But of course your overhead will also differ. You also have to factor in things like insurance.
The other thing that will have an effect is experience and background. A teacher with many years experience, especially within a market where they have built name recognition will be able to set higher rates than someone breaking in. A teacher with training and certifications recognized in their own industry as top notch will often command higher rates. If you are just starting out you may need to set rates at a slightly lower end, and/or offer introductory specials, and/or take some work in established studios to learn the business and make a name.